1. Technical Field
The present invention relates to display devices such as head-mounted displays (HMDs).
2. Background Art
Conventional methods for use with display devices such as head-mounted displays (HMD) include a method for performing direct drawing on a retina of a user by performing two-dimensional scanning using a laser beam (hereinafter, described as a laser-scanning method)(for example, see Patent References 1 and 2). The display devices according to the laser scanning method is also known as: retinal scanning display, retinal irradiation display, retinal direct-draw display, laser scanning-type display, direct-view-type display, retinal scanning display(RSD), virtual retinal display, and so on.
In addition, the laser scanning methods includes a method for achieving three-dimensional display by causing an internal focus changing unit to change the depths of pixels in display of the pixels (for example, see Patent Reference 3).
The laser scanning methods further include a method for causing a wavefront curvature changing unit to change a wavefront curvature radius of a laser light so as to correct the wavefront curvature radius to a target value (for example, see Patent Reference 4).
Each of FIGS. 34A and 34B shows an exemplary structure of an eyeglass-type HMD. The HMD shown in FIGS. 34A and 34B has a frame equipped with: light sources 101 and 110 which emit laser beams; wavefront shape changing units 102 and 109 which control the wavefronts of the laser beams; and scanning units 103 and 108 which perform two-dimensional scanning using the laser beams. The laser lights are projected onto the lenses by the scanning units 103 and 108, are reflected by the deflecting units 104 and 107 provided on the surfaces of the lenses, enter user's eyes to form images on the retinas of the eyes. Here, a half mirror or a hologram optical element (HOE) is used as each of the deflecting units 104 and 107, so as to allow the user to watch both landscape in the external world and images drawn using the laser lights at the same time. In addition, used for each of the scanning units 103 and 108 is a mirror device which performs two-dimensional scanning using a laser light by oscillating a single-plate mirror in a uniaxial or biaxial direction.
In addition, used as another embodiment of a conventional micro-display-type HMD is a structure in which a micro-display such as a liquid display and an organic EL display is used as a light source instead of a laser light source, and a deflecting unit guides light from the micro-display to a user's eye.
As in the case of common personal-computer display screens, eye strain is a problem in visual display terminal (VDT) operation using the above-mentioned HMD. Eye strain is mainly caused when the focuses of the eyes of a user are fixed on a display screen. FIG. 35 shows the structure of a human eye. FIG. 35 is a cross-sectional view of the eye. As shown in the illustration, a human eye visually recognizes a video obtainable when incident light 1205 passes through a crystalline body 1202, and then is condensed on a retina 1204 which is the depth wall of an eyeball 1201.
A human eye changes the thickness of a crystalline body 1202 by relaxing and tensing a ciliary body 1203 composed of muscles. This adjustment is made so as to successfully condense incident light 1205 on the retina 1204.
In general, a person tenses the ciliary body 1203 so as to make the crystalline body 1202 thicker when looking at a close object. Making the crystalline body 1202 thicker shortens the focal length of the crystalline body, which makes it easier to condense near light on the retina 1204. This allows the person to clearly watch a near object 1301. FIG. 36 illustrates this mechanism.
In contrast, the person relaxes the ciliary body 1203 so as to make the crystalline body 1202 thinner when seeing a distant object. Making the crystalline body 1202 thinner lengthens the focal length of the crystalline body, which makes it easier to condense distant light on the retina 1204. This allows the person to clearly watch a distant object 1301. FIG. 37 illustrates this mechanism.
In a VDT operation, the person sees light displayed on a near display screen. Normally, the person tenses the ciliary body 1203 so as to make the crystalline body 1202 thicker. A long-lasting VDT operation keeps the ciliary body 1203 tense. This strains the ciliary body 1203, resulting in eye strain.
To prevent eye strain, the micro-display-type HMD utilizes a function of changing the position of a micro-display or lens (for example, see Patent Reference 5).
A change in the position of a micro-display or lens changes a viewing distance (a distance between a user's eye and a virtual image of a displayed video) of a video presented to the user. Thus, when the video is displayed at a location distant from the user, the user relaxes the ciliary body 1203 so as to make the crystalline body thinner. Utilization of this action makes it possible to mitigate eye strain of the user who uses the HMD.
In addition, the beam scanning-type HMD can perform similar processing by changing the curvature of the wavefront of a beam instead of moving the position of the micro-display. In general, light wavefront curvatures become greater as the lights become distant (a light from an infinite distance is a parallel light having an infinite wavefront curvature). Therefore, an increase in the wavefront curvature of light from a laser light source lengthens the viewing distance of the video to be presented to the user. As a result, the tension of the ciliary bodies in the user's eyes is mitigated.
The image display devices used as HMDs belong to one category of image display devices among mobile display terminals for individual use. In view of wearability, a generally applied structure is an eyeglass structure as shown in FIGS. 34A and 34B.
The image display devices such as HMDs include a known image display device which performs two-dimensional scanning using a laser light assuming that, for example, a part corresponding to a lens in the eyeglass structure is a screen, and performs direct drawing on the retina of an eye of an observer so as to display an image (for example, see Patent Reference 6). Here, a reflection mirror composed of a back surface reflection mirror or a front surface reflection mirror as a pupil transmission lens corresponding to a screen is made into a Fresnel lens. This reduces the thickness, size, and weight of the whole optical system, thereby increasing wearablity.
In the case of an HMD according to a laser scanning method like this, an optical path from a laser light source to a retina of a user through a screen is spatially different for each pixel in the case where the laser light source and a laser scanning unit are arranged at the temple of the user instead of in front of the eye or in the case where a large image is displayed by increasing an angle of view. In other words, the incidence angles, reflection angles, the wavefront shapes and spot sizes of the beams at the deflecting unit such as a mirror disposed on the optical path are different between pixels significantly. Accordingly, even when the laser light source emits beams each having the same wavefront shape and spot size, the beam characteristics such as the beam wavefront shapes and spot sizes are different between the pixels in an image when the beams reach the eye of the observer. The resulting problems are that the sizes of the respective pixels vary, and that the sizes of some of the pixels exceed an allowable range.
Proposed in order to solve the problems is an image display device, according to a laser scanning method, which includes a curvature correcting unit which corrects the wavefront curvature of a laser light to a target value in association with a change in the position of an optical unit, which deflects the laser light to be used for scanning performed on the observer's eye, included in the image display device (for example, see Patent Reference 4). This helps the observer to recognize a precise image having corrected optical characteristics even though the variation degree of optical characteristics such as wavefront curvatures of the laser lights vary depending on the irradiation positions of the optical units on which the laser lights are irradiated.
Proposed in relation to the image display devices such as HMDs which are mounted on user's heads and display images include: an image display device including, as an image display unit, a pixel-type display device such as a liquid element and an organic EL; and various methods such as a method for performing direct drawing on user's retinas through two-dimensional scanning using laser beams.
The image display device like this is required to be small and light as a whole in order to reduce the burden in mounting or wearing the device, so that the user can use it for a long time. Further, if such device is implemented as having a design similar to that of a general eyeglass, the user can perform activities constantly wearing it as in the case of wearing the general eyeglass.
However, it becomes difficult to reduce the size and weight of an image display device according to the method using a pixel-type display device while increasing the image quality and visual field angle. This is because such increase necessitates use of a large ocular optical system including a display unit and a prism and a half mirror which guide lights from the display unit.
In addition, it is difficult to implement such ocular optical system in a general eyeglass-type image display device. It is likely that such ocular optical system must be implemented in a goggle-type or helmet-type image display device rather than an eyeglass-type image display device in order to be placed in front of the user's eyes, and thus that a feeling of comfort in wearing the device cannot be expected.
On the other hand, the retina scanning-type display according to the laser scanning method as shown in FIGS. 34A and 34B is characterized by composing a very small display device using a small micro-electro mechanical system (MEMS) mirror device.
Further proposed is configuring, in an eyeglass form, the device having a thin ocular optical system including a hologram mirror instead of a prism and a half mirror (for example, see Patent Reference 2).
Each of FIG. 38 and FIGS. 39A and 39B shows an example of an eyeglass-type image display device like this.
In FIG. 38, the eyeglass-type image display device 81 includes an eyeglass lens 82, a lens frame 83, and a temple 84 as in a general eyeglass.
A laser beam 86 is irradiated through an aperture 85 in the temple 84, is used for two-dimensional scanning, and is deflected at the image reflection area 87 on the lens 82 in the direction toward the eye 88 of the user. The laser beam 86 entering the eye 88 forms a spot on the retina, resulting in an image to be recognized.
Depending on the structure, an external device 89 including a scanning unit and an electric power unit is connected wired or wireless (represented by broken lines in the illustration). In combination with a speech reproduction device, earphones are further provided (not shown in the drawings).
FIG. 39A is a plan view of the eyeglass-type image display device 81, and FIG. 39B is a side view of the same. Each of the drawings shows only the right part of the user's head and the eyeglass-type image display device 81. In the case of a binocular, the binocular has a bilaterally symmetric structure (this is also true of image display devices to be described later).
In this embodiment, as shown in FIGS. 39A and 39B, the device includes the temple 84 mounting therein a light source 91 which emits a laser beam 86, a scanning unit 92 which performs two-dimensional scanning using the laser beam 86, and a control unit 94 which controls the respective units. Here, the horizontal dimension W of the temple 84 is the minimum width required to arrange the respective elements internally along a virtual straight line. In the case where the diameter of the laser beam which enters the MEMS mirror is approximately 2 mm, the minimum width can be roughly estimated to be 5 to 10 mm.
The vertical dimension H can be roughly estimated to be 30 mm since the eyeglass lens 82 has a height of 25 to 35 mm.
The laser beam 86 is projected toward the eyeglass lens 82 by the light source 91, is reflected by the deflecting unit 93 which is a hologram mirror formed on the surface of the eyeglass lens 82, enters the user's eye 88, and forms an image on the user's retina. The hologram mirror is a photopolymer layer on which a Lippman volumetric hologram is formed. The hologram mirror has a wave selectivity, and thus reflects only the laser beam having a particular wavelength. As a result, the user can visually recognize both landscape in the external world and the image drawn by the laser beam at the same time.
In the case of using a MEMS mirror as the scanning unit 92 in the aforementioned structure, the optical axis for irradiating a laser beam from the ear side of the temple 84 to the MEMS mirror becomes approximately parallel to the center axis of the eye, and the incidence angle α of the laser beam to the MEMS mirror becomes approximately equal to the incidence angle β of the light from the MEMS mirror to the deflecting unit 93 (the incidence angle α is an angle formed by the normal of the reflection surface and the axis of the incident light). Here, α=β=60 degrees is satisfied when the MEMS mirror is disposed as shown in FIG. 39A such that the laser beam from the MEMS mirror is irradiated on the deflecting unit without being shielded by the user's face.
In addition, another example proposes a device having the same structure, but the incidence direction of a laser beam is different (for example, see Patent Reference 7).
In Patent Reference 7, a laser light source unit is provided at the side of an eyeglass lens instead of the ear side so as to guide the laser light to the scanning unit. In reality, the optical path is formed as shown in FIGS. 40A and 40B because there is little space in the part ranging from the scanning unit to the eyeglass lens.
FIG. 40A is a plan view showing an exemplary eyeglass-type image display device structured like this, and FIG. 40B is a side view showing the same.
The scanning unit 92 is disposed at the same position as in the structure shown in FIGS. 39A and 39B. In contrast, the laser beam 86 irradiated from the light source 91 is irradiated from a portion between the eyeglass lens 82 and the scanning unit 92 using folding mirrors 95 and 96. In this case, the incidence angle α to the scanning unit 92 is represented as α=β/2=30 degrees.
[Patent Reference 1] Japanese Patent Publication No. 2932636
[Patent Reference 2] Japanese Unexamined Patent Application Publication No. 10-301055
[Patent Reference 3] Japanese Patent Publication No. 3103986
[Patent Reference 4] Japanese Unexamined Patent Application Publication No. 2004-191946
[Patent Reference 5] Japanese Patent Publication No. 3148791
[Patent Reference 6] Japanese Unexamined Patent Application Publication No. 2000-221441
[Patent Reference 7] Japanese Unexamined Patent Application Publication No. 2003-029198